The invention relates to coiled tubing and, in particular, to preperforated coiled tubing.
Conventional down-hole oil and gas drilling and production techniques require solid casings or liners which maintain the integrity of a well and contain certain drilling fluids. Referring to FIG. 7A, when drilling is complete and the casing or liner 102 is in place, the casing or liner 102, or tubing (not shown), is used to produce hydrocarbons from the pay zone 100 to the surface 101. As a result, the casing 102 must be pierced at this location to allow hydrocarbons to flow into and up the casing 102. This can be accomplished by lowering high energy shaped charges or bullets 104 into the well and firing them through the casing into the formation. However, piercing the casing in this manner contaminates, and sometimes damages, the formation.
Alternatively, referring to FIG. 7B, the casing 102 may be preconditioned in certain areas to selectively allow production through the wall of the casing 102. According to one known type of preconditioning, holes 106 are drilled into the casing 102 before the casing is lowered into the well. Plugs 108 are then placed into the holes to prevent oil or gas from prematurely entering the casing. When the casing 102 is finally positioned in the well and hydrocarbons are to be produced from an area above the pay zone 100, the plugs 108 are removed from the holes 106 either by grinding or by dissolving with a chemical agent.
A disadvantage of conventional perforation methods is that it is necessary to drill a large number of holes in the round walls of the casing. This task is labor intensive and very expensive. In addition, conventional plugging techniques are prone to undesired leakage.
In recent years, coiled tubing has been used in lieu of, or in addition to, conventional casings or liners during oil and gas drilling and production operations. Referring to FIG. 8, coiled tubing 110 comprises a long length of metal tubing on a spool 112. The tubing can be wound and unwound into the well, thus eliminating the need to piece together sections of straight pipe. In order to produce hydrocarbons from the well, coiled tubing must be pierced with bullets or shaped charges, as described above.